The present invention relates particularly to Direct Current electrical wiring systems such as used in Photovoltaic energy installations.
Because Photovoltaic systems are relatively new to the mass market, electrical code requirements in the USA for their installation tend to be excessively cautious, raising cost and hindering adoption. For example, the DC wiring from a solar array is required by the National Electrical Code (NEC) to be contained in metal conduit after it enters a building. Normally, metal conduit is not used in residential wiring, and can be hard to retrofit. The NEC is published by the National Fire Prevention Association, which is supported by the insurance industry, and the caution is based on fear of fire caused by electrical faults in houses of wood-frame construction.
In an earlier version of the NEC, a requirement was introduced mandating the use of Arc Fault Circuit Interruption devices (AFCIs) on some residential AC circuits, due to concern about potential fire risk from Arc faults. The 2008 NEC extended the requirement for AFCIs to substantially every room of the house except those locations already using ground fault breakers (GFCIs). Arc fault breakers are between 20 and 100 times the price of ordinary breakers and thus the cost of the electrical installations for new construction increased substantially. It is however hoped that future increases in production and sales volume of AFCIs will bring down their price. The problems of spurious nuisance tripping caused by, for example, vacuum cleaners, use of arc welders or Amateur Radio transmitters and suchlike in the same neighborhood, remain.
In addition, the 2011 NEC imposed a new requirement (art. 690.11) for Photovoltaic systems which requires that, as well as using metal conduit to contain potential arcing, the DC wiring for PV systems over 80 volts shall employ DC AFCIs, despite there being no such device invented at the time of publication.
It is therefore an objective of this invention to disclose a low cost technique for the detection of wiring faults, so that wiring faults that could potentially lead to arcing are detected and mitigated, and arcing is prevented.